I have managed to get this working this evening by following this thread:
https://archlinuxarm.org/forum/viewtopic.php?f=65&t=14672I'm using the microSD card as /boot and a USB SSD as /
1. edit /etc/mkinitcpio.conf and add pcie_brcmstb to MODULES
i.e MODULES=(pcie_brcmstb)
2. run mkinitcpio -P
3. edit /boot/boot.txt and add the new root partition to root=
e.g. root=/dev/sda1
4. run ./mkscr
The problem I have now is that the ethernet rarely works, I have to reboot several times before It does...